The RI50 is inner-rotating (its rotor is inside its stator) and the U8 is outer-rotating.
With the small gap radius of the RI50, this difference plays an important role that common geometric analyses do not capture.
Specifically, it is often assumed that the rotor with radial thickness \(t_r\)  and stator with radial thickness \(t_s\)  (Fig. REF ) are thin rings, i.e., \(t_r \ll r_g\) , \(t_s\ll r_g\)  [1]}.
While this assumption is approximately true for the rotor, it is not for the stator in the case of the RI50.
This fact means that the stator can hold more copper,
lending it a higher \(K_M\) 
than an outer-rotating motor of the same gap radius and length [2]}.
Such an outer-rotating motor would have \(\sim 4 \times \)  less volume available for the stator.
This effect is captured by \(S_M\)  metric (REF ).
